1. Field
The disclosed embodiments generally relate to fabricating semiconductor devices. In particular, the disclosed embodiments relate to preventing overlay shift and avoiding phase edge formation during fabrication of a chrome-less phase shifting mask.
2. Brief Description of Related Developments
When producing small structures for semiconductor devices, alternating phase shift masks or chrome-less phase shift masks structured by quartz etching can be used in the photolithographic process.
The alternating phase shift masks or chrome-less masks use a transparent pattern formed on or in a transparent mask substrate to create a phase difference that produces a photoresist pattern.
The fabrication of chrome-less phase shifting masks generally involves two or more mask writing steps. The mask writing steps can include writing a phase layer, which includes a main die and PCM, and a chrome layer, which includes fab structures, such as for example, alignment marks and overlay boxes.
Referring to FIG. 1 for example, a mask substrate 10 is formed 100 that includes a layer 12 of a transmitting material, such as for example, quartz, and a layer 14 of a light block mask, such as for example chromium or chrome. The mask substrate 10 can be formed in any suitable process. A layer 16 of a radiation sensitive material, or photoresist, is formed or deposited over the light blocker mask 14 in any suitable manner. The substrate 10 can be divided into a main die or pattern region 10A and a scribe region 10B. The main pattern region 10A is used to form the pattern photomask. The scribe region 10B is used to fabricate alignment and overlay marks.
The overlay control between the writing steps in the mask formation is critical and different. For example referring to FIG. 1A, a mask writing step generally comprises forming 102 a pattern, in a first photolithographic process, on the main pattern portion 10A of the mask substrate 10. The light blocking mask 14 of the mask substrate 10 is then etched 104 using the main pattern portion 10A as an etch mask. This generally involves removing the chrome from the unmasked areas. The substrate 10 is then etched 106 again using an etching process that is capable of etching the silicon oxide layers, equivalent to the quartz layer 12 comprising the mask substrate 10.
To pattern the scribe region 10B of FIG. 1B, the mask substrate 10 is recoated 108 with a second layer 18 of resist and exposed to a second photolithograph process 110, which generally involves using a tool that is different from the tool used in the first photolithographic process 102. The exposed areas of the chrome layer 14 are removed 112 in a suitable etching process, and the structure is cleaned to remove any particles produced by the etching process.
The two step process of FIGS. 1A and 1B causes significant issues with respect to overlay control between the writing steps. In the above process, the overlay control is generally inconsistent and difficult to control. It would thus be advantageous to be able to fabricate a chrome-less phase shifting mask while preventing an overlay shift between the formation of the phase layer and the chrome layer
Another drawback with the conventional alternating or chrome-less phase shifting mask is the creation of residual photoresist features, or phase edge formation after development of the photo resist. A photoresist feature can generally be described as a discrete feature formed by exposing and developing photoresist. The photoresist feature is surrounded by exposed wafer surfaces after development of the photoresist. Residual photoresist features, or phase edges, are generally formed at phase transition boundaries. An example of the formation of a phase edge in a chome-less phase shifting mask process is shown with respect to the one step process flow of FIGS. 2A and 2B.
Referring to FIGS. 2A and 2B, in a first photolithography step 200, the resist layer 22 of the mask substrate 20 has been patterned, on both the main pattern region 20A and the scribe region 20B. In an etching process 202, the exposed regions of the light blocking mask 24 are removed. Then, the remaining regions of the photoresist layer 26 are removed 204. In another etching process 204, the exposed areas of the mask transmitting layer 22 are removed.
As shown in FIG. 2B, a second photo resist layer 28 is formed 206 on both the main die 20A and scribe regions 20B. The portion of the resist layer 28 on the main die region 20A is then exposed in a second photolithography step 208. This leaves only a portion of the resist layer 28 on the scribe region 20B. The remaining portions of the chrome layer 24 are then removed 210 from the main die region 20A in a suitable etching process. The remaining portions of the resist layer 28 in the scribe region 20B can be removed in a resist strip and clean process 212. This leaves the chrome structures 24 on the scribe region 20B.
However, a phase edge defect 29 will exist at the juncture 28 of the main die region 20A and scribe region 20B. This phase edge defect 29 is unwanted and will require an additional photolithography step to remove the phase edge. A discussion of the elimination of phase edge formation is discussed in U.S. Patent Pub. No. 20050221200, the disclosure of which is incorporated herein by reference in its entirety.
It would be advantageous to be able to prevent overlay shift and avoid phase edge formation during fabrication of a chrome-less phase shifting mask.